The invention relates to a knee endoprosthesis comprising a tibia part, a femur part having two condylar surfaces, and a meniscus part arranged between the femur part and the tibia part, the meniscus part having on its upper side two bearing shells for receiving and mounting the condylar surfaces of the femur part and having on its underside a meniscus-bearing surface which rests in a displaceable manner on a tibia-bearing surface on the upper side of the tibia part.
A knee endoprosthesis of this kind is described in, for example, U.S. Pat. No. 6,013,103. In this previously known knee endoprosthesis, the tibia-bearing surface is of flat configuration and rests on the flat upper side of the tibia part. The displacement of the meniscus part relative to the tibia part takes place as pure parallel displacement of the meniscus part parallel to the plane of the tibia-bearing surface and the upper side of the tibia part. In unicondylar knee endoprostheses, too, it is known to mount a meniscus part, which in this case has only one bearing shell for the mounting of a femoral condyle, with a flat meniscus-bearing surface on a likewise flat tibia-bearing surface (EP 1 584 309 A1).
In knee endoprostheses of this kind, the meniscus part can be displaced and also rotated towards various directions relative to the tibia part, with the rotation taking place, as a rule, about the center of the meniscus part. In some cases, peg-shaped guides are provided for this purpose (U.S. Pat. No. 6,013,103). It is, however, also known to mount the meniscus part on the tibia part such that a rotation takes place about an axis of rotation which is offset in a medial direction in relation to the center of the meniscus part (U.S. Pat. No. 6,013,103). To achieve this, special guides, for example, lateral walls, must be provided on the tibia part for the meniscus part.
In practice, it has been found that with knee endoprostheses of such construction the anatomical relations can be reproduced relatively well, but difficulties may arise due to this special construction limiting the attainable bending angle between femur and tibia, as the meniscus part, which moves in only one plane relative to the tibia part, restricts the movement of the bones. In the case of high bending angles, the bones may strike the meniscus part, thereby restricting the bending angle.
The object underlying the invention is to so construct a generic knee endoprosthesis that in addition to optimum reproduction of the anatomical characteristics of the healthy knee joint, it allows, in particular, a high bending angle.